This disclosure relates generally to Micro-Electro-Mechanical Systems (MEMS) structures, including tilting mirror devices used in optical components and optical communication networks.
Optical components that switch, attenuate, filter, and process optical signals are widely deployed in optical networks, typically in the 1550 nm or 1310 nm wavelength windows. In many of these optical components, a rotating or tilting mirror is used as a beam-steering element. In these optical components that use tilting mirrors, Micro-Electrical-Mechanical System (MEMS) devices are often used to implement the rotating mirror or tilt-mirror element. In some prior art embodiments, the MEMS rotating mirror or tilt-mirror is fabricated from silicon using semiconductor processing methods and equipment. The mirror can be fabricated as an integral part of the silicon device structure, and is often coated with a thin metallic layer (or layers), using gold, aluminum, or some combination of metal layers to achieve high reflectivity at the appropriate wavelengths.
Prior art optical components that use a rotating or tilting MEMS mirror as a beam-steering element are described in U.S. Pat. Nos. 6,838,738 and 6,628,856, as well as in other patents and literature. The tilt-mirror MEMS structure may have a single rotational axis, or two rotational axes. The moving portions of the MEMS structure are typically suspended from the fixed portions of the structure by structural elements that act as torsion springs or beams. The torsion beams support the moving parts of the structure, and also provide a spring force that resists the tilting or rotating of the moving portions of the structure. In some prior art embodiments, electrostatic actuators are used to tilt the mirror, utilizing multiple groups of comb-finger structures, fabricated in two or more layers of silicon. The MEMS structures may be fabricated from single-layer or double-layer Silicon-On-Insulator (SOI) wafers, with one or two buried oxide layers. In some embodiments, the fabrication process results in a void, or empty space, underneath the rotating or tilting mirror.
The system requirements placed on optical components typically include requirements for resisting external shock and vibrational forces. These external shock and vibrational forces may cause unwanted rotation of tilting of the MEMS mirror, creating a variety of optical impairments. In order to minimize the effects of external shock and vibrational forces, it is desirable for the torsion beams that support the moving portions of the MEMS structure to be as stiff as possible. At the same time, the spring constant of the torsion beams must be low enough to allow sufficient rotation or tilting of the mirror, around one or two axes of rotation, to meet the optical requirements of the component, taking into account the amount of rotational force that can be provided by the electrostatic actuators.
In prior art embodiments where the moving portions of the MEMS structure is supported by torsion beams, acting as springs, the rotational or tilting motion of the mirror will have a resonant frequency for each axis of rotation, determined by the mass of the moving portion of the structure, and the spring constant of the torsion beams. External shock or vibrational forces, as well as intentional movement of the MEMS mirror, may result in oscillatory movement of the MEMS mirror, especially at the resonant frequencies.